Merge build-utils-meson/{diagnostics,threads} into build-utils-meson/common

This reduces the amount of boilerplate. More importantly, it provides
a place to add compiler flags (such as -O3) without having to add it
to every subproject (and the risk of forgetting to include it).

build-utils-meson/common/meson.build

@@ -1,3 +1,10 @@
+# This is only conditional to work around
+# https://github.com/mesonbuild/meson/issues/13293. It should be
+# unconditional.
+if not (host_machine.system() == 'windows' and cxx.get_id() == 'gcc')
+  deps_private += dependency('threads')
+endif
+
 add_project_arguments(
   '-Wdeprecated-copy',
   '-Werror=suggest-override',

build-utils-meson/threads/meson.build

@@ -1,6 +0,0 @@
-# This is only conditional to work around
-# https://github.com/mesonbuild/meson/issues/13293. It should be
-# unconditional.
-if not (host_machine.system() == 'windows' and cxx.get_id() == 'gcc')
-  deps_private += dependency('threads')
-endif